In order to convey a large quantity of heat that is generated in a semiconductor or electronic equipment, a method has been developed to obtain a high cooling performance by bonding a material having a high heat conductivity to the outer unit of a semiconductor, and flowing a refrigerant through the inside thereof as a heat absorber. A method has also been developed that obtains a higher cooling effect by boiling the refrigerant with the heat absorber. In order to dissipate the heat that the refrigerant has snatched to the outside, it is necessary to cause the refrigerant to circulate between the heat absorbing unit and a heat radiating unit. Generally, a pump is employed for circulation of the refrigerant.
In the case of an ebullient cooling system, there has been proposed a thermal siphon-type cooling structure that, by installing a heat absorbing unit in the lower portion of the cooler and heat radiating unit in the upper portion, does not require a pump by utilizing the principle of vapor collecting upward with respect to the direction of gravity due to the difference in density between the vapor that is generated and the liquid. For example, this cooling structure is disclosed in Patent Document 1 and Patent Document 2. Patent Document 1 provides a siphon-type ebullient cooler for cooling a power semiconductor. This ebullient cooler is characterized by a structure in which the heat absorbing unit and the heat radiating unit are formed comparatively integrated, in addition to a pump not being required. In this ebullient cooler, there is a need to integrally mold the structure of the heat absorbing unit that is in contact with the heat generating body and the heat radiating unit and a circulation path of the refrigerant. For that reason, this ebullient cooler is expensive when miniaturizing it for electronic components, and so using it for various purposes is difficult.
Patent Document 2 provides a method that performs circulation of a refrigerant by installing a condenser above a boiling unit to produce a siphon effect. However, in an electronic apparatus, it is not always possible to extend a pipe upward and install the condenser due to the internal layout. Also, in the case of installing the pipe that is to be connected to the condenser vertically upward from an evaporator, it is unavoidable for condensation to occur in the pipe. In this case, a liquid membrane is formed in the pipe, and a flow is generated with it trying to return to the boiling unit due to gravity. This not only becomes resistance to the vapor that is heading toward the condenser, but also reduces the cross-sectional area of the evaporation path, thus producing pressure loss. As a result, not only is the performance of the condenser not exploited, but its operation as a cooler becomes unstable. Moreover, in the worst case, there is the risk of drying out of the evaporator being induced.
The structure disclosed in Patent Document 3 is the same as the structure disclosed in Patent Document 2. That is to say, in the structure of Patent Document 3, since the condenser is provided vertically above the evaporation heat absorbing unit, there is the destabilizing factor as was described in the above Patent Document 2. The structure of Patent Document 3 can be inexpensively produced since the refrigerant flow passage from the condenser to the evaporating unit and then to the condenser is formed by bending back a single pipe. However, this kind of structure is not suitable for the flow of refrigerant of both the liquid phase and the gaseous phase. Also, in the evaporator, heat reception is not performed over the entire heat generation surface.
In order to solve this issue, Patent Document 4 provides a structure that separates the liquid phase flow path and the gas phase flow path by making the tubing into a two-layer structure. With this structure, it is possible to improve the circulation characteristic of the refrigerant, while maintaining the characteristic of being able to install the heat radiating unit separate from the heat receiving unit. That is to say, it is possible to reduce the pressure loss between the heat radiating unit and the heat receiving unit, leading to an improvement of the cooling characteristic. All of the aforementioned proposals are attractive cooling methods in the case of there being an element having a dominant power consumption within the electronic device. However, in the case of there being a plurality of heat generating elements, a plurality of these coolers becomes necessary.
Patent Document 5 provides a structure that cools a plurality of heat generating parts, being a cooler that utilizes phase change. Heat receiving units of a number equal to the elements to be cooled are used, with the heat receiving units and the heat radiating units being constituted by a series fluid circuit. In this structure, the refrigerant that has vaporized by the heat received from the heat generating elements on the upstream side passes the heat receiving units of the elements on the downstream side. Since it is necessary for a liquid phase refrigerant to be supplied to the downstream side to cool the downstream side elements, a structure has been proposed that forcibly circulates the refrigerant with a pump. By using a pump, for example it is possible to change the flow rate in accordance with the amount of heat generation. However, as a cooling structure, it becomes complicated and expensive. Moreover, since the liquid phase refrigerant is forcibly supplied, during the heat movement from the elements to the refrigerant in the heat receiving unit, liquid cooling that does not accompany the phase change and ebullient cooling that accompanies the phase change are mixed. The heat-transfer property of ebullient cooling is higher. For this reason, increasing the ratio of ebullient cooling is desirable for improving performance. For that reason, Patent Document 5 is characterized by heating the refrigerant just before entering the heat receiving unit so as to put it in a state of easily undergoing phase change. The structure for heating makes the structure of the cooler more complicated and expensive. Moreover, there is the problem of the load on the heat radiating unit becoming unnecessarily large.
Patent Document 6 provides a cooler that targets a plurality of heat generating elements. This cooler is a structure that supplies refrigerant that has been cooled by a heat radiating unit to each heat receiving unit through pipes that are arranged in parallel, in order to optimize the cooling of each heat generating element. In this cooler a liquid cooling system is presumed, and so circulation of the refrigerant is needed for each circuit installed in parallel. The refrigerant flows back to the heat radiating unit that is aggregated into one element. The heat radiating unit is designed so as to dissipate the entire quantity of heat that is collected, but the heat dissipation efficiency is worse compared to the case of dissipating heat without the heat radiating units being aggregated.